1. Field of the Invention
The present invention relates to a power source apparatus provided with a high capacity array of batteries to obtain both high output voltage and high output current.
2. Description of the Related Art
A power source apparatus with both high output voltage and high output current has many batteries (or battery cells) connected in series to increase the output voltage. In this type of power source apparatus, the series-connected batteries are charged by the same current and discharged by the same current. Accordingly, if all the batteries have exactly the same characteristics, no imbalance (between batteries) will develop in battery voltage or remaining charge capacity. However, as a practical matter, batteries cannot be manufactured with exactly the same characteristics. With the repetition of charge-discharge cycles, battery voltage and/or remaining charge capacity imbalance will develop between batteries. Further, battery voltage imbalance can cause over-charging or over-discharging of a specific battery (a battery with low capacity or high internal resistance). To avoid these detrimental effects, a vehicle power source apparatus has been developed with cell balancing capability that detects the voltage of each battery and eliminates imbalance between battery cells.
Refer to Japanese Laid-Open Patent Publication 2006-149068.
As shown in FIG. 4, the vehicle power source apparatus 40 cited in JP 2006-149068 has a discharge circuit 42 connected in parallel with each battery 41 in the series-connected battery array. Each discharge circuit 42 is a series-connection of a discharge resistor 43 and a switching device 44. In this power source apparatus 40, a battery 41 that has become high in voltage is discharged through its discharge circuit 42 to restore cell balance and eliminate disparity between batteries 41. A discharge circuit 42 lowers the voltage of a specific battery 41 by switching the switching device 44 ON to discharge the battery 41 through the discharge resistor 43.
This power source apparatus 40 can eliminate imbalance between the series-connected batteries 41. In this type of power source apparatus, high output voltage can be attained by connecting many batteries in series, but the current capacity is that of the individual batteries. Since the power output of a power source apparatus is proportional to the product of voltage and current, increasing the current can further increase the power output. Specifically, by connecting many batteries in series and parallel, both the output current and output voltage can be increased to further increase the power output of the power source apparatus. In a power source apparatus with many batteries arrayed in series and parallel, voltages of the series-connected batteries can be equalized to eliminate imbalance in the same manner cited in JP 2006-149068. However, it is necessary to charge and discharge many batteries to control equalization of the voltages of all the batteries and that equalization process is extremely time consuming. For example, a high output power source apparatus with 1000 lithium ion batteries connected in series and parallel has the drawback that equalization of the voltages of 1000 lithium ion batteries requires time.
The present invention was developed considering the types of prior art problems described above. Thus, it is a primary object of the present invention to provide a power source apparatus that can efficiently equalize the battery cells while connecting many cells in series and parallel.